1. Field of the Disclosure
The present disclosure relates to a convergent-divergent nozzle; in particular, but not exclusively, to an adjustable convergent-divergent diffuser e.g. for a gas turbine engine.
2. Description of the Related Art
Convergent-divergent nozzles (con-di nozzles) are known. They can be used to meter air flow in a conduit containing the con-di nozzle. The flow accelerates as it moves towards the ‘throat’ of the nozzle, and when it achieves the speed of sound the mass flow cannot increase further for a given inlet pressure and temperature. The flow is then decelerated downstream to allow the dynamic head to be recovered so that high pressure gas is delivered to the target zone.
Fire zones are common on aero-engines. They are designated to be any zone that contains both ignition sources (such as hot surfaces or high power/voltage electrical systems) and flammable fluids/fuels (such as aviation fuel or oil) that might leak from units or pipes within the zone thus leading to a fire. This may be one definition of a fire-critical zone. Regulations dictate that these zones must be ventilated with fresh air to minimise the possibility that a fire might occur by reducing the ‘fuel’ to air ratio. Thus, an alternative or additional definition of a fire-critical zone may be a zone that requires a predetermined (for example by regulation) flow rate of ventilation fluid during use. These zones often contain electrical and mechanical units that are sensitive to high temperatures and so the ventilation also has the function of cooling them.
In aero-engines, conduits such as inlet scoops are commonly used to take air from a ‘mainstream’ flow and direct it into a particular zone such as a fire zone, or towards some device that uses the fluid, via a conduit. These conduits typically incorporate a known con-di nozzle, and are sometimes referred to as convergent-divergent diffusers (con-di diffusers).
Airworthiness regulations dictate five volume changes of air per minute are required to minimise the possibility of fire in a fire zone in an aero-engine. This necessitates a relatively large inlet scoop, and convergent-divergent nozzle with a large throat are designed into the system to achieve the required air flow at low altitude where the aircraft forward speed is low. Consequently, at high altitude this can produce more than forty air changes per minute. This is not problematic for the fire zone, but the excess air flow represents an aerodynamic inefficiency since the air contains a large dynamic head which is lost. This aerodynamic inefficiency leads to higher fuel consumption.